This is a really neat method to include CSS styles into an HTML email inline, which is apparently the best practice for HTML emails.
Instead of tediously adding inline styles Mark Guinn wrote a class that extends SilverStripe's Email class and injects CSS rules inline automatically using Emogrifier.
This is awesome because you can keep the seperation between presentation and content, and simply integrate a stylesheet into an HTML email automatically just before sending. As a basic example without changing Mark's code at all:
$email = new ProcessedEmail(
$from = '',
$to = '',
$subject = '',
$body = ''
$css = file_get_contents(Director::getAbsFile($this->ThemeDir().'/css/Email.css'));
'InlineCSS' => "<style>$css</style>"
Then just include $InlineCSS in the email template and the styles will be made inline automatically when ProcessedEmail-> parseVariables() is called. Very cool!